Repulsion motors have torque speed performance curves and other performance characteristics similar to those of series DC motors and universal series motors, and also offer high starting torque with comparatively low starting current, and the ability to sustain the high starting torque for long periods of time. This type of motor is also adaptable to a wide range of controllable speeds, wherein the no load speed of the repulsion motor can be many times higher than the synchronous speed. Repulsion motors are typically constructed with a stator having field windings energized by single phase AC power to provide a magnetic field between a pair of stator poles. A DC rotor is provided with an armature winding connected to a commutator, wherein brushes were previously provided to ride on the commutator for shorting specific rotor winding coils as the rotor turns. Providing AC power to the stator field winding creates a stator field, causing current to be induced in the shorted coils of the armature to create a rotor field that interacts with the stator field to produce torque and rotation of the armature. This type of repulsion motor has not been widely adopted because the brushes and commutator wear out quickly due to arcing and heat generated by the brushes contacting the commutator. An alternative design is known as a repulsion start, induction-run motor, in which a squirrel cage rotor is provided in the wound repulsion motor rotor armature, together with mechanisms for lifting the brushes from the commutator when the rotor speed reaches a predetermined value, whereafter the motor runs as a normal induction motor. This hybrid type motor provides the high starting torque advantages of repulsion motors with the low maintenance advantages of induction motors.
A fundamental improvement in motor design and control has resulted from the introduction of brushless repulsion (BLR) motors, which provide the performance advantages of repulsion motors without the need for brushes. Examples of BLR motors are shown in Haner U.S. Pat. No. 5,424,625; Haner U.S. Pat. No. 6,049,187; Haner U.S. Pat. No. 6,108,488; and Jones U.S. Pat. No. 6,321,032, which are incorporated by reference so that background information and structure of brushiess repulsion motors need not be described in detail. In the brushless motors of Haner U.S. Pat. No. 5,424,625, the brush-based commutator circuits are replaced with rotor mounted electronic switches controlled by optical detectors or sensors for selectively shorting individual rotor coils in response to optical signals received from light emitting diode (LED) light sources positioned on the stator. The stator poles are energized to create a stator magnetic field, from which flux passes directly through the rotor and induces a voltage in each shorted rotor coil. When a rotor coil switch is closed, current flows through the coil to produce flux, torque, and rotation. However, when the switch is open, current cannot flow and no torque or rotation is produced by that coil. Closing the switch for a longer period produces more power and increases the speed. Thus, the motor's torque and speed can be adjusted as desired by controlling how long (over what rotational angle) the switches are open. The rotor coil switches are activated by an optical signal detector as the detector rotates past the stator-mounted LEDs, and the LEDs can be turned on for varying amounts of time to adjust the motor's speed and the torque produced by the motor.
In the BLR motors of Haner U.S. Pat. No. 5,424,625, the LEDs are positioned at predetermined locations within a small arcuate portion or sector of the stator circumference and are illuminated to activate rotor-mounted electronic circuits to short rotor coils when the coils are in a predetermined angular position relative to the stator poles. In particular, the stator field creates alternating positive and negative torque sectors or regions in which shorted rotor coils therein produce positive and negative torque, respectively. Thus, for rotation in a given direction, it is desirable to short the rotor coils when they are within the stator field torque sectors of the polarity corresponding to the desired rotational direction. The LEDs of conventional BLR motors are typically controlled in closed loop fashion according to a measured or sensed rotor position and a desired motor operating condition, such as speed, torque, etc. Moreover, in conventional BLR motors, the optical signal detectors are mounted in-line with the torque producing segments of the corresponding rotor coils (i.e., the angular location of each light detector is at the same angular center as the associated rotor coil segment), and the control LEDs are positioned at predefined angular locations on the stator that correspond to torque producing regions in the stator field. For instance, Haner U.S. Pat. No. 5,424,625 shows a two pole BLR motor with six rotor coils, where each rotor coil has two torque producing segments parallel to the rotor axis. In this two pole motor design, torque of a first rotational polarity (e.g., positive torque) is produced when a shorted rotor coil segment is within one of two diametrically opposite positive torque sectors of the stator field, where each of the two positive torque sectors occupies approximately 90 mechanical degrees extending from a soft neutral angle along a line extending between the stator pole centers and a hard neutral angle on a line between the centers of the gaps between the stator poles. Ninety degree wide negative torque sectors are located between the positive torque sectors, wherein one or more sets of diametrically opposed LEDs are placed on one side of a magnetic hard neutral location of the stator field, wherein several sets of such diametrically opposite LED pairs can be used in the Haner style BLR motors to control the angle at which rotor coils are shorted within a given positive or negative torque sector.
In BLR motors, as with other motor types, it is desirable to provide smooth rotor torque to drive a given load. The smoothness of motor torque is typically measured in terms of ripple torque, where the peak torque is often much greater than the average torque, with the torque value varying as a function of rotor angle. In this regard, the conventional two pole BLR motors may also suffer from significant torque ripple, where the peak torque occurs as a shorted rotor coil passes near a stator pole tip. Where multiple rotor coils are used, the torque peaks of individual coils are separated in time and combine to produce a ripple effect in the total produced torque, which can lead to vibration, noise, premature bearing wear, and other problems in BLR motors. One method of reducing this torque fluctuation is to provide a large number of stator poles. Thus, while Haner U.S. Pat. No. 5,424,625 illustrates a two pole BLR motor, it may be desirable to provide multiple stator pole pairs to achieve smoother torque than is possible in 2 pole BLR motors. In addition, a relatively large number of coil segments may be provided in the rotor of a BLR motor in order to achieve a given set of speed and torque performance parameters.
For precise control of rotor speed and/or torque, moreover, it is important to be able to selectively short one or more selected rotor coils at a given point in time without inadvertent shorting of other non-selected coils. However, as the number of stator poles and the number of torque-producing rotor coil segments are increased in conventional BLR motors, the spacing between the stator mounted LED signaling sources and the spacing between the rotor-mounted detectors decreases. This may lead to crosstalk situations in which non-selected rotor coils are shorted when the associated optical detector is proximate a lighted LED. Because the detector is aligned with the corresponding rotor coil in conventional BLR motors, the angular spacing between neighboring optical signal detectors on the rotor of Haner U.S. Pat. No. 5,424,625 and other prior BLR motors is equal to the spacing between rotor coil segments. Light dispersion effects in LEDs and other optical signaling devices become more pronounced as the motor dimensions are reduced and as the number of stator poles and rotor coil segments increase, thereby making the manufacture of BLR motors with consistent operational performance more difficult. One manufacturing difficulty is variation in LEDs, in which light provided by a given LED establishes a corresponding light distribution or pattern (cone), with distributions varying according to LED type, manufacturing production lot, vendor, orientation, current supply, age, temperature, and LED-to-detector distance. These production variations in LED signaling source patterns make it difficult to provide precise illumination spots such that the rotating detectors and corresponding switching circuitry of the rotor are activated at exactly the desired time and place without undesired activation of detectors associated with non-selected rotor coils. Such difficulties may be addressed by using more coherent light sources or fiber optic devices, but these solutions are costly. Accordingly, there is a need for improved brushless repulsion motors, by which rotor coil switching circuits can be actuated at precise angular positions and times in a cost effective manner, and torque ripple can be reduced in BLR motors, particularly for motors having multiple stator pole pairs and relatively large numbers of rotor coil segments.